1. Field of the Invention
The present invention relates generally to devices used to interconnect the structural members of a building for the purpose of transferring forces between structural members such as the walls of a building and its roof framing systems, and more particularly to a set of high capacity hardware components that can be used to seismically retrofit existing tilt-up buildings, and the like, having long span timber trusses so as to substantially improve the interconnection between the building walls and roof framing system.
2. Description of the Prior Art
All man-made structures and particularly "tilt-up" buildings of the type frequently used in commercial and industrial environments can be subjected to excess natural and abnormal forces, i.e. seismic wind blast, etc., with disastrous consequences. Investigations have found that such tilt-up buildings especially those with timber roof framing systems, are vulnerable to damage and/or collapse during earthquakes. These types of buildings typically consist of a structure that is constructed with concrete wall panels that are pre-cast horizontally on the ground, and after curing, are tilted into place to form the wall units of a typically rectangular enclosure. Roof framing systems are then affixed to and at least partially supported by the wall panels.
The roof framing systems of older tilt-up buildings that were built between the early 1950's (when the initial construction of tilt-up buildings began) and the mid 1960's were generally constructed with long span timber roof trusses and timber roof joists. The timber trusses of these buildings were normally oriented to span the short direction of the building with spacing between the trusses generally varying between 16 and 24 feet. The roof joists generally consist of 2.times.8's, 2.times.10's or 2.times.12's spaced at 24 inches o.c. and span between the timber trusses. At the perimeter of the building the roof joists span between the timber trusses and the tilt-up wall panels, where they are usually framed onto a timber ledger that is bolted to the wall panels. Roof sheathing for these buildings is normally comprised of 3/8 inch or 1/2 inch plywood.
After the mid 1960's the roof framing systems of most tilt-up buildings were constructed with glulam beams instead of long span timber trusses and a "panelized" roof framing system instead of roof joists. These modifications to the roof framing systems were typically made for economic reasons. A panelized roof framing system consists of timber purlins, timber sub-purlins (also known as stiffeners) and roof sheathing. The roof sheathing normally consists of 4.times.8 sheets of 3/8 inch or 1/2 inch plywood and spans between the sub-purlins. The sub-purlins are generally 2.times.4's or 2.times.8's and span between the purlins. The purlins typically consist of 4.times.12's or 4.times.14's and span between the glulam beams (or in some cases long span timber trusses). The plywood sheathing is usually oriented with its long dimension parallel to the sub-purlins or perpendicular to the purlins. The sub-purlins are generally spaced 24 inches apart. The purlins are typically 8 feet apart to accommodate the length of the plywood sheathing. The glulam beams are often spaced 20 to 24 feet apart and sections of the panelized roof are typically fabricated on the ground and raised into place with a crane or a forklift.
In areas subject to high seismicity, the connection between the concrete wall panels of most older tilt-up buildings and their roof framing systems is inadequate for the currently established seismic design standards for such buildings. Generally this connection consists of only the nailing between the roof sheathing and the timber ledger that is bolted to the wall panel, and relies on a mechanism that subjects the ledgers to "cross-grain bending", a mechanism that is highly vulnerable to failure. The deficiency associated with this type of connection was responsible for numerous failures and collapses of tilt-up buildings during the 1971 San Fernando earthquake. As a result, this type of connection has been specifically disallowed since the 1973 edition of the Uniform Building Code was published. It is generally recommended that tilt-up buildings with such deficiencies be retrofitted with new connections per the currently established seismic design standards and/or recommendations for such buildings. For tilt-up buildings with long span timber roof trusses there has not been a uniform method of installing retrofit structural elements for the purpose of connecting the wall panels to their roof framing system.
One of the connection problems often encountered is improperly dealing with the steep and varying angles present between the roof framing system and the wall panels (such problems being generally not encountered in tilt-up buildings with glulam beams and panelized roof framing systems). At the present time, the retrofit installation of these connections is usually undertaken with "hold-down" connection devices and/or custom fabricated hardware components that incorporate currently available structural components such as rods, devices and turnbuckles.
"Hold-down" connection devices were initially developed to provide a means of attaching the studs at the ends of plywood shearwalls to foundations or other studs, but have been adapted for various other uses. In tilt-up buildings with long span timber roof trusses, hold-downs are sometimes used to attach the wall panels to the roof joists. Due to the slope of the roof joists in these buildings such attachments are usually difficult to install and do not properly resolve both the horizontal and vertical force components associated with the attachment.
Hold-down connection devices are generally designed for overall load capacity without regard to overall device deflection. Such connection devices are subject to excessive deflections when loaded. Recent studies of earthquake damage to buildings have recommended that the device deflection of connection devices used in wall attachments be limited in order to prevent the forces associated with these attachments from effectively being transferred through the plywood roof sheathing, and thereby subjecting the timber ledgers to cross-grain bending. Some building departments have taken these recommendations into account and have established their own device deflection criteria. One such criteria put forth by the city of Los Angeles has reduced the allowable capacity of hold-down devices in general by a factor of 3 to 5. Such criteria increases the size, number and costs associated with hold-down connection devices. Another deficiency of the hold-down connection device is that the bolt holes bored through the wood members are prone to being oversized, especially in paired applications where access is impaired. Connection devices installed with such bolt holes must undergo an additional amount of deflection before the "slop" between the bolt and the wood is taken up. Such deflections affect the structural performance in a building in a manner similar to the deflections associated with a hold-down connection device.
Custom fabricated hardware components that incorporate elements such as rods, devices and turnbuckles are subject to a number of drawbacks, problems and deficiencies. In general, these components must usually be designed by an engineer and be fabricated on a limited basis, and thus take time to procure, and are expensive. Furthermore, devices and turnbuckles are usually cast items and are subject to brittle failure. The current seismic design philosophy is to avoid the use of any hardware element, component or mechanism that is subject to brittle failure. The allowable load capacity of clevices and turnbuckles is thus low, and in many cases is lower than that of the rod. Thus, these elements usually establish the allowable load capacity of the hardware component or connection. Moreover devices and turnbuckles are not necessarily "readily available" (particularly clevices) and can be difficult to procure. In fact, devices and turnbuckles can be quite expensive, especially when their allowable load capacity is considered. For rod tightening purposes, turnbuckles are usually provided with right-hand (standard) threads at one end and left-hand (reversed) threads at the other. This generally requires that a special length of rod be provided with right-hand threads at one end and left-hand threads at the other. These special lengths of rod can be difficult to procure, are expensive and add an additional element of complexity to the hardware component or connection.